Generally, a method for impregnating a blowing agent into resin granules that are obtained by suspension polymerization is widely adopted as a method for manufacturing thermoplastic resin expandable granules. However, this method has a problem in that a waste water disposal plant is required for disposing a large quantity of waste water containing a suspension stabilizer, a surface active agent, and a polymerization initiator or the like since the polymerization is carried out in a water medium. Furthermore, the method has a problem in that the yield of expandable granules having a predetermined granule size is low because the granules obtained by the method vary in size.
As a method for solving these problems, a manufacturing method called an extrusion process, in which a resin melted and mulled in an extruder is extruded from a die that is fitted to the end of the extruder so that the resin is cut into granules, is known.
The extrusion process is classified into two types according to the timing of cutting the resin into granules. One is a method called a hot-cut method, in which the resin is extruded as a string shape from the die into pressurized liquid and cut directly by a rotary cutter attached to the die. The other is a method called a cold cut method, in which the resin is extruded once in the air as a string shape and then brought into coolant liquid, and the string-shaped resin is cut after cooling while being drawn up from the coolant. In comparison to the latter (the cold cut method), the former method (the hot-cut method) has the advantages in that the productivity of the expandable granules is high and the obtained granules are easily handled because spherical granules that have no edges can be obtained.
For example, a method for manufacturing the thermoplastic resin expandable granules by the hot-cut method has been proposed in which a thermoplastic resin and a blowing agent that has a boiling point of −50° C. to 0° C. are mulled in an extruder, extruded into water of 20° C. to 100° C. and equal to or less than 40 atmospheres, and cut into granules directly in the water (e.g., refer to Patent Document 1).
In addition, a method has been proposed in which a thermoplastic resin and a blowing agent are melted and mulled in an extruder, the resin is extruded into heated and pressurized liquid, and the resin is immediately cut into granules. Subsequently, the granules are cooled slowly in a pressurized container, the pressure in the container is released, and aging the granules under temperature conditions equal to or greater than 40° C. and equal to or less than a temperature 15° C. higher than the low temperature endothermic peak determined by DSC measurement (e.g., refer to Patent Document 2).
In addition, as a method for manufacturing thermoplastic resin expandable granules in which, unlike the processes described above, thermoplastic resin granules that do not contain a blowing agent are manufactured by a hot-cut method and a blowing agent is subsequently impregnated into the resin granules to manufacture thermoplastic resin expandable granules, a method has been proposed in which, when the thermoplastic resin is extruded from a nozzle die into water and cut by a cutter blade that rotates while in close contact with a die surface to form a spherical shape, the resin temperature is adjusted such that the viscosity of the melted resin at a die entrance is 100 to 50,000 poise and the extrusion rate per nozzle hole is 0.1 to 6.0 kg/hr (e.g., refer to Patent Document 3).
Alternatively, as a manufacturing method for spherical expandable polystyrene resin granules in which polystyrene resin granules containing a conjugate diene polymer are impregnated with a blowing agent in a water medium, a method has been proposed in which, prior to the impregnation of the resin granules with the blowing agent, the resin is melted in an extruder, extruded through extrusion holes, and cut. In this method, the shearing speed of the resin in the die land part is equal to or greater than 2,500 sec−1 and equal to or less than 10,000 sec−1, and the apparent viscosity is equal to or greater than 150 poise and equal to or less than 700 poise (e.g., refer to Patent Document 4).
Patent Document 1: Japanese Examined Patent Application, Second Publication No. S48-20423
Patent Document 2: Japanese Unexamined Patent Application, First Publication No. H07-314438
Patent Document 3: Japanese Unexamined Patent Application, First Publication No. S61-195808
Patent Document 4: Japanese Unexamined Patent Application, First Publication No. H09-208735